Overreacting yes or no - porn in relationship

So my boyfriend and I have been together for 2 years. Our daughter is 6 months old. He knew from before we got together that I wasn’t comfortable with either of us watching porn in a relationship& that I view it as a form of cheating

Well last night he admitted he watches it while I’m on my period and when I was recovering from having our daughter. It hurts even more because I would ask him about it randomly if the topic came up and he would always swear he never watched it. Now he’s saying he does so it’s all been a lie. It makes me sick to even think of him hiding to watch those things in the bathroom or when I’m asleep. Especially right after I had our daughter.

He says I’m too insecure and that he never met anyone that thinks it’s a big deal. He says no other girl cares if their boyfriend watches it: I told him to me it’s cheating. The only thing in the way is a screen so it’s like the same as if the girl was there doing it in person. I told him I don’t want to be with him if he was going to continue and especially lie. I feel horrible and don’t know what to do. Am I overreacting?